SYNTHESIS AND STRUCTURE OF BIMETALLIC COMPLEXES WITH SIGMA,PI-BRIDGEDMONOCARBENE LIGANDS

The Fischer method was employed to synthesize bimetallic monocarbene c omplexes from lithiated [Cr(SC4H4)(CO)(3)] and metal hexacarbonyls. In addition to the formation of [M{C[eta(5)-C4H3SCr(CO)(3)]OEt}(CO)(5)] (M = Cr 1 or W 2) the complexes [M{C[eta(5)-C4H3SCr(CO)(3)]O(CH2)(4)OE t}(CO)(5)] (M = Cr 3 or W 4), were also. obtained. The formation of 3 and 4 is ascribed to the activation of tetrahydrofuran (thf) by a sigm a,pi-co-ordinated acyl metalate, which triggers the cleavage of the th f ring and leads to-its inclusion into the alkoxy substituent of the r esulting carbene moiety, In polar solvents the Cr(CO)(3) fragments are displaced to give known pentacarbonyl[ethoxy(thienyl)carbene] complex es and the new mononuclear carbene complexes [M{C(C4H3S)O(CH2)(4)OEt}( CO)(5)] (M = Cr 5 or W 6). The analogous benzo[b]thienyl carbene compl ex [Cr{C[eta(6)-C8H5SCr(CO)(3)]OEt}(CO)(5)] 7 was prepared similarly, but did not afford the thf-inserted product and the corresponding conv ersion into the mononuclear carbene complex [Cr{C(C8H5S)OEt}(CO)(5)] 8 , was much slower.